1. Field of the Invention
This invention relates to methods for the synthesis of diamond and diamond-containing coatings on substrates by chemical vapor deposition (CVD) and to the resulting coated substrates. These coatings have superior adhesion to the substrate and are useful in many industrial applications.
2. Description of the Background
Crystalline diamond is one of the most remarkable substances ever discovered. It has the highest atom number density of any of the terrestrial materials and is harder than any other solid, strongly resistant to abrasive wear, chemical wear, thermal shock, and degradation. The diamond also has the lowest compressibility and the highest thermal conductivity, nearly four times that of copper. Classically, the term diamond is reserved for crystalline cubic carbon. The cubic structure or lattice of the carbon atoms of the diamond is derived from a visualization of the saturated six member ring structure in layers, stacked and residing in a (111) coordination. A lattice with hexagonal symmetry is referred to as Lonsdaleite. Both cubic and hexagonal carbon lattices are comprised entirely of sp.sup.3 tetrahedrally coordinated carbon. Compounds containing significant amounts of sp.sup.2 trigonally coordinated carbon atoms are technically not diamond, but diamond-like phases. The term diamond as used herein will not carry this distinction, but is used to refer to both the true diamond and the diamond-like forms of carbon.
Diamonds have become indispensable in industrial applications and the present sources of natural diamond do not equal existing demands. These demands are currently being met with synthetic diamonds. In the early 1900's there were a number of fairly credible reports of the synthesis of diamonds. Today these reports are generally discounted as either fanciful or just plain wrong. It was not until 1955 that the methods used to produce synthetic diamonds were first published (H. Liander, ASEA Jl. 28:97-98, 1955; F. P. Bundy et al., Nature 176:51-55, 1955). These researchers used a carbon solvent/catalyst to break the so-called graphite-diamond equilibrium line, which separates the two major atomic forms of the carbon atom, and formed sp.sup.3 hybridized diamond from sp.sup.2 hybridized graphite. Their methods required sustained temperatures of greater than 1600K and sustained pressures of greater than 60 kbar (6.0 GPa). Although the synthetic diamonds created were crude compared to their natural counterparts, these researchers demonstrated that the process was possible and took the subject matter out of the sphere of alchemy and into the modem world and opened a whole new field of technology.
Presently, over 90% of the worldwide demand for industrial diamonds is met by synthetic diamonds. There is hardly a production facility in modem industry that does not directly or indirectly require the synthetic diamond. Some of its most common uses are as optical windows for infrared and visible transmissions, as wear resistant super-hard cutting tools, as super-sharp edges in medical applications, as abrasive, grinding materials and all tribological applications, as sliding materials, and as thin films for high temperature, high power semiconductor devices. As might be expected, the number and variety of uses to which synthetic diamonds are being applied is increasing and the demand is growing exponentially.
The diamond is a crystal like any other and the basic principles of crystal growth apply to its synthesis. Growth is limited by the basic thermodynamic variables of temperature, pressure and composition. The method selected for diamond growth depends upon what type of diamond is desired. Although there are as many different crystal structures as can be imagined, there are two basic types of diamond crystals, perfect or nearly perfect (Saw diamond abrasive (SDA)) and imperfect (Resinoid diamond abrasive (RDA)). The conventional method of diamond synthesis uses a carbonaceous material, usually graphite, which is mixed and compacted with a suitable solvent such as nickel, cobalt, or iron. The mixture is compressed to over 60 kbars in a reaction vessel and heated to above the eutectic melting point of the solvent. When this point is reached, diamond crystals begin to grow and eventually precipitate from the solution. The is the so-called high-pressure, high-temperature (HPHT) method of diamond synthesis.
The actual molecular processes which provide diamond growth are largely unknown as are the identities of all of the molecular species involved, but a few observation can be made. There is a large activation energy barrier between the two phases of carbon, graphite and diamond. Because the molar entropy of graphite is greater than that of diamond, the thermodynamic instability of the diamond increases with increasing temperatures. Consequently, increasing temperatures require increasing pressure to bring graphite and diamond into equilibrium. In all synthetic methods, increased temperatures are required solely for kinetic purposes. In high pressure diamond synthesis, molten carbon must be produced to create the high diffusion rates necessary to achieve crystal growth.
The conventional HPHT method of diamond synthesis is slow, arduous, and very expensive requiring a great deal of expertise and dedicated bulky machinery. Intensive efforts around the world have been aimed at developing cheaper and more useful forms of synthetic diamond and particularly diamond film. Diamond films impart all of the advantageous properties of diamonds such as increased abrasive and chemical wear resistance, thermal shock resistance, and resistance to degradation, to a substrate. The advantages are clearly enormous. The principle difficulties have been encountered in determining how best to attach and secure the film to the substrate. Over the past twenty years, significant progress has been made in growing diamond film by chemical vapor deposition (CVD). This method overcomes the need for extremely high pressures and the associated bulk and expense of the apparatus required to achieve these pressures. Further, the scope of materials which can be coated with diamond has been greatly expanded. Potential substrate materials are no longer required to withstand the enormous temperatures and pressures of the HPHT process.
The basic CVD process involves the introduction of one or more gasses into a reaction vessel or chamber. To the chamber is added sufficient energy to excite the gasses and raise the energy level of the individual gas molecules. As the energy level rises, some of the molecules ionize. At the substrate surface, large numbers of free radicals form and there is a high degree of atomic mobility which is necessary to achieve diamond growth. As the energy level falls, precursor carbon molecules enter a metastable state forming a diamond-like lattice structure which deposits on the nearest surface. The metastable state refers to the stable formation of diamond structures within the graphite stabilization region of the carbon temperature/pressure stabilogram. The metastable phase depends on selected conditions including the types of gasses and deposition surfaces, the ratio of gasses in the mixture, the presence of contaminants or additives, and the temperatures of the substrate and of the reaction. It is believed that, at least in part, metastable phases form from high energy diamond precursor molecules which become quenched upon reacting with the substrate. Metastable phases form from precursor molecules with high chemical potential which accounts for the need for a hydrocarbon source. Some of the more useful hydrocarbon sources are acetylene, allylene, ethane, isobutane, isobutylene, pentane, trimethylene, isopentane, propylene, butane, methane, ethylene, and propane.
There are a number of different CVD techniques which are categorized by the particular methods in which the source gas is decomposed or, in other words, the hydrocarbon molecules are raised to a higher energy level. Although there are various combinations and modifications, the basic CVD techniques are, hot filament CVD, generally described in U.S. Pat. No. 4,938,940, microwave plasma CVD, generally described in U.S. Pat. No. 4,767,608, electric discharge plasma CVD, generally described in U.S. Pat. No. 5,110,405, and combustion flame CVD, generally described in U.S. Pat. No. 5,135,730. The disclosures of these U.S. patents are hereby specifically incorporated by reference.
In hot filament CVD, a mixture of hydrocarbon and hydrogen gasses are passed over a tungsten filament or foil in a reaction chamber. To the chamber is added a substrate such as graphite. The filament is energized and the gasses ignited heating the substrate to between 600-1200K. The oxygen/acetylene ratio (R) is preferably between 0.75 and 1.2. Reaction pressure is selected from the range of 60 to 760 Torr. The dissociation products at these temperatures and pressures consist mainly of the radical species C, C.sub.2, CH, CH.sub.2, CH.sub.3, and atomic hydrogen, as well as unreacted gasses. Deposition rates, which are rather slow, may be related to the enhanced recombination rate of atomic hydrogen or other radicals. Filaments are placed within about one centimeter of the substrate surface to minimize thermolization and recombination between radicals.
Although one of the more well investigated methods hot filament CVD does have drawbacks. Radiation heating of the substrate can produce excessive surface temperatures and more importantly, non-uniform surface temperatures. With excessive substrate temperatures, carbon radicals fail to form on the surface. With low surface temperatures, carbon fails to deposit on the substrate in any form. Consequently, with a non-uniform temperature distribution across the substrate surface carbon deposition is uneven and the resulting film quality is poor. Additionally, as with all methods which require an enclosed chamber, substrate size is limited to chamber size.
In microwave CVD, initial nucleation rates are high in comparison to filament-type CVD methods which allows for lower substrate temperatures. As before, a reaction chamber is required and into the chamber is injected a mixture of hydrocarbon and hydrogen gasses. To these gasses is applied a microwave or other high-frequency discharge to raise the molecules to a higher energy state. In the resulting plasma are growth nuclei such as diamond or inorganic powders. As the energy level falls to achieve the metastable state, diamond crystals form on the particles. Using this method, substrate surface temperatures as low as 823K have been reported (A. Sarabe and T. Inuzuka, Appl. Phys. Lett. 46:146, 1985). This method is highly reproducible and most useful for the creation of single-crystal diamonds. However, there is little to no binding between individual diamond particles and as with most forms of CVD, substrate size is limited to chamber size.
Diamond growth has also been reported using DC discharge between an anode and a cathode. In this method, inorganic crystals are placed into a reaction chamber containing an anode, a cathode, and a reaction gas including at least one organic compound. A direct current discharge between the two electrodes produces a plasma heating the chamber to between 800.degree. and 1100.degree. C. The inorganic crystals which may be made from silicon carbide or boron nitride are vibrated and single crystal diamond is deposited on the particles. Although growth rates are high there are numerous drawbacks. First, the DC discharge produces a bombardment of ions, electrons, and neutral gas particles with large amounts of energy. In order to withstand this bombardment, the elements within the reaction chamber must be made of a refractory metal such as molybdenum. A filament is often required to start and/or maintain the DC discharge which must also be made of a chemically stable material such as tungsten. Both molybdenum and tungsten are quite expensive. Moreover, the entire reaction is performed in a vacuum at around 10.sup.-7 Torr which adds a level of complexity to every step of the entire process.
Each of these methods involve thermally controlled diamond synthesis wherein the temperature of the reaction is much higher than the temperature of the substrate. These higher reaction temperatures produce dissociation of the carbon source gas. However, the extent of dissociation and the gas phase chemistry are unique, making the role of particular excited states in each method nearly impossible to assess. Substrate temperature, although lower then reaction temperature, must still be sufficiently high to allow for mobility of surface molecules, particularly hydrogen for the saturation of carbon atoms. Also, elaborate apparatus made of expensive metals are typically required as is a reaction chamber to either maintain a vacuum, to provide a rarefied atmosphere of nobel gasses, or to totally eliminate certain compounds from the potential reactants. Moreover, these methods are more useful for the creation of single crystal diamond, not for the synthesis of diamond film. Adherence is not considered and intercrystal bonding does not take place to any significant degree.
In overcoming some of these problems, combustion flame CVD has proved to be a significant advancement. Using hydrocarbon gasses in the open air, sufficiently high temperatures are achieved to ionize precursor carbon molecules which fall into the metastable state as the energy dissipated and form diamond films on most any substrate. Combustion flame CVD starts from nearly equimolar mixtures of oxygen and usually acetylene although other hydrocarbon gasses have been tested. Oxygen is added directly or as an integral part of the hydrocarbon gas. Hydrogen gas prevents surface reconstruction and suppresses the formation of unsaturated carbon nuclei. Combustion temperatures of about 2500.degree. C. to about 3000.degree. C. have been achieved. Carbon is partially dissociated from the source gas at higher temperatures, but total ionization is not believed to be critical to the process. In the presence of atomic hydrogen, the diamond surfaces are likely to be saturated with hydrogen because H--H bond energy is greater then C--H bond energy. Also, atomic hydrogen suppress the formation of graphitic nuclei. The energy for these exothermic reactions is supplied by the energy added to dissociate hydrogen (H.sub.2 .fwdarw.2H). Because of the dynamic interaction between atomic hydrogen and the diamond surface, there is a steady state concentration of free surface sites which is continually being replenished by carbon from the source gasses.
In general, cutting or welding torches have been useful in combustion flame synthesis. Gas flow is maintained to create a flame from a single outlet port. Flame size is adjusted by altering the flow of gasses to create a feather in the flame which is the area of incomplete combustion and the zone where diamond deposition occurs. Into this feather region is placed a substrate to be coated. The substrate temperature is maintained at a constant by altering its exact position in the flame and by forcing a cooling fluid past the opposite end of the substrate, which may be integral to the substrate support structure. Substrate surface temperature must be maintained between 300.degree.-1200.degree. C. and is preferably between 800.degree.-900.degree. C.
Diamond deposition is limited to a ball-like region on the substrate. At high carbon to oxygen ratios deposits with high amounts of amorphous carbon are found. At low carbon to oxygen ratios there is no carbon deposited due to complete oxidation of the components. When the carbon to oxygen ratio is set to about one, diamond films are observed over the entire deposition area. However, over this area, deposition is not uniform. At the edges, the density is non-uniform, decreasing at increasing distances from the deposition center. Generally the diamond crystals are well formed cubo-octahedrons of a high density and relatively free of pin holes. Consequently, although diamond films formed by combustion flame CVD are of a fairly high and consistent quality, strengths are low and deposition times are usually quite long.